Emulsion types and Emulsion Theories

Emulsions are mixtures of two or more immiscible liquids, such as oil and water, stabilized by the addition of an emulsifying agent.

Basis of emulsion classification:

Emulsion can be classified into two types on the following two points

  • nature of the dispersed phase.
  • and continuous phase.

Theories of emulsions

Following theories describe  the different types of emulsions:

Oriented wedge theory

This theory explain the formation of water-in-oil (W/O) emulsions.

This theory has following main points :

  1. The emulsifying agent used in the preparation has both hydrophobic and hydrophilic ends,letting it to interact with both immiscible liquids.
  2. The emulsifying agent is added to the mixture of water and oil, and it position itself at the interface between the two liquids.
  3. The emulsifying agent containg two ends
  • Hydrohobic end showing attraction towards the oil phase.
  • while the hydrophilic end is attracted towards the water phase.
  1. The emulsifying agent, arrangement at the interface forming a (wedge-like structure) giving stability to emulsion.
  2. The wedge-like structure decrease the interfacial tension between the two liquids, letting them to fuse and form a stable emulsion.

Draw back of oriented wedge theory:

  • Farther types of emulsions, such as oil-in-water (O/W) emulsions, are not described by the oriented-wedge theory.


Overall, the oriented-wedge theory provides a basic understanding of the mechanism behind the formation of W/O emulsions and is widely accepted in the field of emulsion science.

phase inversion theory

This theory describe the formation of oil-in-water (O/W) emulsions.

Main points of this theory is given below:

  1. An emulsifying agent is used to add in mixture of oil and water.
  2. Effect of concentration of emulsifying agent:
  • At small concentrations of the emulsifying agent, it is favourably adsorbed at the oil-water interface, resulted in formation of water-in-oil (W/O) emulsion.
  • As the concentration of the emulsifying agent is increased, it begins to adsorb more at the water-oil interface.
  1. Inversion point:

At a critical concentration of the emulsifying agent, known as the inversion point.

following things can be happened on inversion point

  • The emulsion undergoes a phase inversion
  • and becomes an O/W emulsion.
  1. The inversion point depends on various factors such as
  • the nature of the emulsifying agent
  • the concentration of the emulsifying agent
  • and the properties of the immiscible liquids.
  1. The phase inversion theory is useful in the design and optimization of emulsion formulations, as it gives information about the factors
  • that affect the stability
  • and type of emulsion formed.


Overall, the phase inversion theory is an essential idea in emulsion science and provides a framework for understanding the formation of O/W emulsions.



Film Droplet Balance Theory

This theory explains the formation and stability of both

  • water-in-oil (W/O)
  • and oil-in-water (O/W) emulsions.

This theory have following main points:

  1. Emulsion droplets are considered as a liquid film that surrounds a core of one of the immiscible liquids.
  2. The film consists of a layer of emulsifying agent that adsorbs at the interface between the two liquids, forming a monolayer.
  3. The monolayer decrease the interfacial tension among the two liquids, allowing them to mix and form an emulsion droplet.
  4. The stability of the emulsion droplet depends on the
  • balance of forces acting on It which includes
  • the interfacial tension
  • the pressure difference across the droplet interface
  • and the thickness of the film surrounding the droplet.
  1. The interfacial tension have a tendency to shrink the droplet, while the pressure difference across the droplet interface have the ability to expand the droplet.
  2. The thickness of the film surrounding the droplet also affects its stability, its thicknes provide greate stability to it.
  3. The film droplet balance theory is helpful in understanding the factors:
  • that affect the stability of emulsions
  • and can be used to optimize the formulation of emulsions.


Overall, the film droplet balance theory provides a valuable framework for understanding the stability of emulsions and is widely used in emulsion science.

Diffusion Controlloed Theory


The diffusion-controlled theory is a theory that gives information about the stability of emulsions based on;

  • the rate of diffusion of the emulsifying agent between the two immiscible liquids.

This theory contain following important points:

  1. Emulsions are stabilized by
  • the adsorption of an emulsifying agent at the interface between two immiscible liquids.
  1. The rate of diffusion of the emulsifying agent molecules between the two liquids governs the stability of the emulsion droplets.
  2. Effect of rate of diffution:

If the rate is too slow

  • the emulsion droplets may coalesce and separate
  • leading to the breakdown of the emulsion.

If the rate is too fast

  • the emulsifying agent may be rapidly depleted
  • leading to the destabilization of the emulsion.
  1. The stability of the emulsion can be optimized by adjusting
  • the emulsifying agent concentration
  • droplet size distribution
  • and other factors to ensure that the rate of diffusion is optimal.
  1. The diffusion-controlled theory is useful in
  • predicting the stability of emulsions
  • and in designing emulsions with specific properties.


Overall, the diffusion-controlled theory provides a valuable framework for understanding the stability of emulsions and is widely used in emulsion science.